The shells of several types of nuts, including walnuts and hazelnuts, have a very dark natural color. Dark-shelled nuts that are harvested for food use are routinely bleached to make the shells much whiter in color and thus more commercially desirable.
Macadamia nuts are normally sold in shelled form. The meats of the shelled nuts are normally a creamy white color. However, the meats of these nuts frequently become stained to a light brown color while the nuts are still in the shell. These nuts, which comprise a significant percentage of the macadamia nuts grown, are commercially undesirable because of their unpleasant appearance and because the stained nuts may discolor foods that are prepared using the nuts. The stained nuts are normally separated from the remaining nuts and discarded.
Various processes have been used for bleaching the shells of nuts.
These processes have enjoyed limited success. For the most part, existing processes are unsatisfactory because they utilize chemicals having toxic emissions, effluents, or residues.
For example, in-shell hazelnuts are currently bleached using an oxidation bleaching process. The shells of the nuts are wetted with water and the nuts are placed on a grate positioned over pots of burning sulfur dioxide. The fumes reach the nuts and react with the water on the shells to form sulfuric acid which bleaches the skins of the nuts. This process produces toxic sulfur emissions and also leaves a potentially hazardous byproduct, sulfuric acid, on the nut shells. Concern over the potentially toxic effects of sulfur residues has led countries in Europe to bar importation of products having a sulfur residue, and regulatory agencies within the United States are beginning to take steps that would prevent use of the sulfur oxidation process due to the environmental hazards created by the sulfur emissions.
Current methods of bleaching walnuts also result in toxic residues and effluents. One method utilizes the application of chlorine bleach to the surfaces of the nuts, a process which has been banned in European countries. Another involves a mixture of hydrogen peroxide and sodium hydroxide--a caustic agent frequently referred to as lye--and produces toxic effluents which, for environmental reasons, are difficult to dispose of. Peroxide processes using lye are strongly alkaline and produce unstable alkaline solutions above pH 9 which require neutralization before final disposal.
To date there is no widely-used method for bleaching the stained meats of shelled nuts. The only currently acceptable solution for discolored nut meats is that of separating the stained nut meats from the unstained ones and discarding them.